I have read that I should set the Thickness to 4 (13mil x .254=3.3 and then rounded up to the next whole number).

and set the Platen Gap to Wide.I am going to print extra long sheets (37 inch) cut off a roll, and although I have tried to flatten them there is inevitably going to be some buckle / curl as I feed it through, and as it gets wet with ink (drying time per head pass set to 25).Should I set the Platen Gap to Wider? What is the drawback to doing so?

I assume Paper Thickness raises the head accordingly. What does Platen Gap do ?

Paper thickness adjusts the transport mechanism to allow for the media, but doesn't change the distance from the head to the paper surface. This places the head at the optimum distance from the surface of the media. The platen gap is what changes the gap between the head and the paper. Widening the platen gap requires the droplets to travel further than they were intended and may reduce the quality of the image slightly, especially on high quality PK ink papers (such as EFP), but can help prevent head strikes.

Thanks Wayne. So Platen gap is like an 'up/down' adjustment between the head and thae paper. But I'm not sure I understand what Paper Thickness does then ?"Paper thickness adjusts the transport mechanism to allow for the media"

That's exactly right. It adjusts the transport mechanism so the paper can go through easily. If it's too tight for the paper, you can have feed problems. That has nothing to do with the distance from the top of the paper to the print head. In my opinion, the texture of the Epson Exhibition Fiber reduces sharpness more than enough to mask any effects of variation in distance to the print head. "5, wide" ought to work fine.

Meanwhile, if you are having problems with marks on the surface of Exhibition Fiber, that may be happening outside of the print area as the paper moves forward. As the ink hits the surface, the paper wants to swell, but the back stays drier and pulls against the swelling front to make the paper buckle into (usually) a sort of smooth "M" shape across the width of the sheet. The peaks of the "M"-shaped buckle may drag against the underside of the area where the paper leaves the print mechanism. So, a way to reduce that buckle is to have the back slightly moist, so as the front picks up ink, it's not being restrained by the back. I've done that with the large sized Harman paper in its earlier version. Later versions don't seem to be as prone to buckling or scratches. You lightly spray water on the back, being careful not to let it transfer to the front. Smooth the water down with a folded sheet of paper towel, and watch the paper buckle upwards in the reverse of the "M", maybe as much as an inch high. Eventually, it will flatten down to within about a quarter inch, after which you can insert it in the printer. Let it sit a while as you make your print driver adjustments, and then print. The slightly moist back will counter the tendency of the paper to buckle. If you live in a really dry area, though, the paper will totally dry out in a hurry, and it might not work as well, especially if you are cutting long panorama sheets.